Full wave simulations provide a valuable tool for studying the spatial and temporal nature of an acoustic field. One method for producing such simulations is the finite-difference time-domain(FDTD) method. This method uses discrete differences to approximate derivatives in the governing partial differential equations. We used the FDTD method to model the propagation of finite-amplitude sound in a homogeneous thermoviscous fluid. The calculated acoustic pressure field was then used to compute the transient temperature rise in the fluid; the heating results from absorption of acoustic energy by the fluid. As an example, the transient temperature field was calculated in biological tissue in response to a pulse of focused ultrasound. Enhanced heating of the tissue from finite-amplitude effects was observed. The excess heating was attributed to the nonlinear generation of higher-frequency harmonics which are absorbed more readily than the fundamental. The effect of nonlinear distortion on temperature rise in tissue was observed to range from negligible at 1 MPa source pressure to an 80% increase in temperature elevation at 10 MPa source pressure.

Research into the behavior of clusters and arrays of fluid or solid particles is made possible by acousto-electric levitation in air and charging of “seed” droplets (10–30 μm in diameter) which come together in 2-D clusters (with up to 300 droplets). Such clusters condense into larger drops (e.g., 50–300 μm) which form uniformly spaced 2-D arrays of monodispersed drops. Similar behavior has been observed for charged solid particles. This research has applications to studies of drop evaporation, combustion,nucleation, and materials synthesis.

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The boundary element method is used to model two-dimensional acoustic radiation and scattering from a body of arbitrary shape above an infinite plane of flat surface and homogeneous impedance. The particularity of the study is the use of an indirect integral representation of the solution, given in terms of the jumps of pressure and its normal derivative through the boundaries. A variational formulation is associated with the boundary indirect integral equations modeling our problem. The major difficulty in the formulation is the infinite feature of the plane, which is avoided by introducing an appropriate Green’s function. Numerical results of the attenuation of sound by noise barriers are presented. They show good agreement with other results in the literature.

The problem of propagation of pulses in the radiating near zone of a large circular normal transducer directly coupled to a homogeneous and isotropic elastic half-space is re-visited. It is shown that for certain observation angles the impulse response approach is computationally inefficient. A new method based on the so-called wavefront expansions of the impulse response is developed instead. The expansions are obtained by the analytical harmonic synthesis of the high-frequency asymptotics of the transducer field. Unlike these asymptotics the wavefront expansions are expressed in terms of elementary functions only. The direct P, edge P and Swaves as well as the transition regions (penumbra and axial region) are described. The uniform asymptotic expansions applicable throughout the radiating near zone are derived as well. The code based on the time convolution of the pressure input function with the wavefront expansions is compared to a direct numerical code. It is thousands of times faster but practically just as accurate except that the phenomena related to the head waves are not described. Formulas pertaining to the far field are also offered.

The source simulation technique or related approaches like the multipole method, the superposition method, etc. are used for calculating the sound field radiated (or scattered) from complex-shaped structures. However, it is known that these techniques can lead to ill-conditioned systems of equations, and their numerical treatment requires extreme care. A new stabilized variant of the source simulation technique—called the full-field method—has been developed by using the exterior instead of the interior Helmholtz integral formulation or, equivalently, by expanding the sound field into special trial and weighting functions. These functions are chosen in such a way that the resulting matrix becomes more diagonally dominant. The full-field method is applied to the acoustic radiation from a pulsating sphere and to the high-frequency scattering from a cylinder and a nonconvex structure. The numerical results are compared with calculations obtained from other methods. It is shown that the improved method leads to better conditioned sets of equations which can be solved directly without singular-value decomposition, since the associated condition numbers are decreased strongly, in some cases by a few orders of magnitude.

Interface conditions at a boundary between two porous media are derived directly from Biot’s equations of poroelasticity by replacing the discontinuity surface with a thin transition layer, in which the properties of the medium change rapidly yet continuously, and then taking the limit as the thickness of the transition layer approaches zero. The interface conditions obtained in this way, the well known “open-pore” conditions, are shown to be the only ones that are fully consistent with the validity of Biot’s equations throughout the poroelastic continuum, including surfaces across which the medium properties are discontinuous. But partially blocked or completely impermeable interfaces exist; these may be looked upon as the case of a thin layer with its permeability taken to be proportional to the layer thickness, again in the limit as layer thickness approaches zero. This approach can serve as a simple recipe for modeling such an interface in any heterogeneous numerical scheme for poroelastic media.

Theoretical expressions for sound radiation from a single incident duct mode, arriving at the open end of a semi-infinite circular unflanged duct with rigid walls, are used to obtain numerical results for (1) the single-mode sound powertransmission coefficient, and (2) the multimode far-field directivity factor. For the multimode calculations the modes are assumed incoherent, and a weighting model is adopted which includes, as special cases, equal power per mode (above cutoff), and excitation by incoherent monopoles or axial dipoles uniformly distributed over a duct cross section. High-frequency asymptotic features of the results are explored in detail and analytical approximations are given. The findings have practical application to sound powermeasurement from tall exhaust stacks.

The dispersion of the first two longitudinal wave modes, and was experimentally investigated for a cylindrical shell (such as a pipe or tube) that was completely filled with a liquid. It was observed that the presence of a liquid inside the cylinder dramatically alters the dispersion curve for the mode by dividing (or branching) the curve into approximately equally spaced regions separated by cutoff-type behavior. This branching was attributed to coupling between the unperturbed mode in the shell and the unperturbed longitudinal modes in a liquid cylinder with rigid boundaries, where N is an integer. The physical mechanism for the mode coupling was determined to be radial resonances in the combined liquid/pipe system. In time domain, the liquid effects on the dispersion are manifested as a long-duration signal or a series of short-duration pulses, depending on the pulse length of the transmitted signal relative to the reciprocal of the frequency interval between branching.

The acoustic field radiated from piezoelectric transducers is usually predicted supposing that the transducer vibrates in thickness mode. However, different reports have shown that not only thickness vibrations were excited, but also plate waves. These waves are responsible for discrepancy between the experimental acoustic fields and those predicted by the Rayleigh integral. It could be supposed that the plate waves are strongly attenuated in piezocomposite materials, as mechanical cross-talk between neighboring elements of the composite structure is fairly weak. A similar effect could be achieved in piezoceramic material by employing a heavy backing, which partially damps the plate waves. These opportunities of plate wave damping are investigated in the present paper. Three transducers are studied, which have identical geometrical characteristics, but are made from different materials. The plate waves in these transducers are indirectly compared by measuring corresponding ultrasound fields and comparing them with theoretically predicted field. It is shown that plate wave patterns are strongly material dependent and that it is only for piezocomposite sources (even when highly focused) that Rayleigh integral modeling can accurately predict the pressure field distribution.

The angular spectrum approach (ASA) is conventionally applied to the evaluation of acoustic fields from planar radiators because it is usually based on the 2-D Fourier transform (or the zero-order Hankel transform in the axisymmetrical case) which is implemented only in a plane. The present paper is intended to extend the ASA to more general cases where radiators have curved surfaces. For this purpose, two approaches are developed. The first one is the extended ASA and is derived in a general way. From this approach, the angular spectrum of a curved radiator is given by a double integral that does not take the 2-D Fourier transform form, and thus cannot be implemented using 2-D fast Fourier transform (FFT) but by numerical integration. The second approach is the indirect ASA that gives the angular spectrum via 2-D Fourier transforming an initial field pre-calculated in a plane. The method for calculating the initial field is proposed based on the method developed by Ocheltree and Frizzell for planar sources. An example is given of a linear array with a cylindrically concave surface, and in this case, the angular spectrum (the double integral) from the extended ASA reduces to a single integral. The angular spectra of the array are calculated using both approaches and compared. The comparison has shown that the angular spectra obtained from both approaches are in excellent agreement. The accuracy and efficiency of the two approaches are studied in the numerical implementation. In this example, the extended ASA has been shown to be more efficient and more accurate than the latter approach. Both approaches can be applied to arbitrarily curved transducers. In the general case where the double integral cannot be reduced to a single integral, the latter approach can be more efficient.

The work of a previous paper on the modeling of scattering by a partially buried cylinder is extended to allow the cylinder to have full elasticity (and not just be a fluid structure as in the previous work). The results of computations showing the effects of increasing burial on the backscattered field, both in the spectral and temporal domains, are given. A shelled and a solid aluminum cylinder are considered for a grazing angle of incidence which is subcritical.

The D.O.R.T. method (French acronym for Decomposition of the Time Reversal Operator) is a scattering analysis technique using an array of transducers. The method is effective to achieve detection and selective focusing on pointlike scatterers through inhomogeneous media [J. Acoust. Soc. Am. 99, 2067–2076 (1996)]. Laboratory measurements in a water waveguide are presented. Taking advantage of the multiple reflections at the interfaces of the guide, high resolution is achieved with the D.O.R.T. method. The separation of two scatterers and the selective focusing are obtained with a transverse resolution at least nine times better than the free-space limit prediction. The detection of a scatterer from the water/air interface of the guide is also achieved with high resolution (1/20 of the free space diffraction spot). The effect on the D.O.R.T. method of surface waves produced at one interface of the guide is measured. Finally, the impulse response function of each scatterer to the array is computed as a combination of the eigenvectors of the time-reversal operator obtained at each frequency. Using these impulse Green’s functions, selective focusing with high temporal and spatial compression is performed.

Assessing the size of cetacean populations in the open ocean has traditionally relied on visual surveys alone. The addition of acoustic monitoring can complement these surveys if reliable protocols can be formulated and calibrated with visual techniques. A study is presented to estimate fin whale population statistics based on near-continuous recording from a single hydrophone. Range to calling animals is estimated by transmission loss and multipath methods to provide a minimum population density estimate. Results are derived from recordings at a hydrophone site north of Oahu, Hawaii that have been the focus of earlier studies. The average calling whale density is 0.027 animals/1000 km2, while the seasonal maximum calling whale density is about three times the average, or 0.081 animals/1000 km2. Over 30 fixed hydrophone sites are available around the Worlds Oceans from which such statistics could be generated.

The purpose of this research is to branch out from thermoacoustics in the plane wave geometry to study radial wave thermoacoustic engines. The radial wave prime mover is described. Experimental results for the temperature difference at which oscillations begin are compared with theoretical predictions. Predictive models often assume a uniform pore size and temperature continuity between the stack and heat exchangers; however, stacks of nonuniform pore size and temperature discontinuities between the stack and heat exchangers are common imperfections in experimental devices. The radial engine results are explained using a theoretical model which takes into account these prevalent construction flaws. Theory and experiment are shown to be in agreement after the complications are included. Spectral measurements show that an additional feature of the radial geometry is the anharmonicity of the resonant modes which significantly reduces nonlinear harmonic generation.

Resonantultrasound spectroscopy was used to measure the orthorhombic elastic constants of rolled, polycrystalline plates of Cu, Cu–5% Zn, and Cu–15% Zn. The experimental results were fit to theoretical expressions to determine the orientation-distribution coefficients and These coefficients are related to texture (the nonrandom orientation of crystallites). The experimental results were in good agreement with theory for the Cu and the Cu–15% Zn materials. The agreement was not as good for the more anisotropic Cu–5% Zn material, especially for the in-plane compressional constants and The ultrasonically derived W’s were compared to those obtained from neutron measurements for the Cu–Zn alloys. Pole plots based on the two types of measurements, using and were in excellent agreement for the 15% Zn material, and in qualitative agreement for the 5% Zn material. The results support the idea that acoustic methods can be used to measure the low-order W’s in polycrystalline materials.

Accurate knowledge of the velocitydispersion of Lamb modes is important for ultrasonicnondestructive evaluation methods used in detecting and locating flaws in thin plates and in determining their elastic stiffness coefficients. Lamb mode dispersion is also important in the acoustic emission technique for accurately triangulating the location of emissions in thin plates. In this research, the ability to characterize Lamb mode dispersion through a time-frequency analysis (the pseudo-Wigner–Ville distribution) was demonstrated. A major advantage of time-frequency methods is the ability to analyzeacoustic signals containing multiple propagation modes, which overlap and superimpose in the time domain signal. By combining time-frequency analysis with a broadband acoustic excitation source, the dispersion of multiple Lamb modes over a wide frequency range can be determined from as little as a single measurement. In addition, the technique provides a direct measurement of the group velocitydispersion. The technique was first demonstrated in the analysis of a simulated waveform in an aluminum plate in which the Lamb mode dispersion was well known. Portions of the dispersion curves of the and Lamb modes were obtained from this one waveform. The technique was also applied for the analysis of experimental waveforms from a unidirectional graphite/epoxy composite plate. Measurements were made both along and perpendicular to the fiber direction. In this case, the signals contained only the lowest order symmetric and antisymmetric modes. A least squares fit of the results from several source to detector distances was used. Theoretical dispersion curves were calculated and are shown to be in good agreement with experimental results.